Method for Applying a Phosphate Coating on a Steel or Iron Part, and Corresponding Steel or Iron Part

ABSTRACT

Method for applying a phosphate coating on an iron or steel part, and corresponding iron or steel part. The method according to the invention consists in a first coating step with a first phosphating, through which a first crystalline layer of iron and manganese phosphates is obtained, an intermediate step of partial pickling of said first crystalline layer of phosphates, leaving the base metal partially uncovered, and a second coating step comprising a second phosphating through which a second crystalline layer of iron and manganese phosphates is obtained, covering the partially pickled first layer of phosphates and the base metal. The method provides a coating with a lower thickness, a finer crystallisation and a lower rugosity Rk than the one that can be obtained by a simple phosphating.

FIELD OF THE INVENTION

This invention relates to a method for applying a phosphate coating on a steel or iron part. The invention also relates to a steel or iron part obtained by said method.

STATE OF THE ART

Precision parts intended to undergo a sliding friction with high contact stress must be provided with an anti-friction coating which guarantees a high mechanical resistance and a low coefficient of friction. Manganese phosphate coatings, due to their high mechanical resistance, provide good results and thus they are commonly used in the finishing of steel or iron parts. Several methods for forming such coatings are known, being of special interest those consisting in immersing the part to be treated in an aqueous solution of manganese phosphate. It is known that by adding oxidation agents to the aqueous solution, finer coatings are obtained. On the other hand, it is known that using manganese phosphate solutions containing nitrate, and wherein the amount of nitrate in the form of NO₃ ⁻ exceeds the quantity of phosphate in the form of PO₄ ³⁻, high density coatings having fine granulation and a thickness lower or equal to 5 μm are obtained. Furthermore, it is known that on iron surfaces, operating with low-density solutions, zinc or manganese phosphate coatings containing at least 1% of nitrate and practically without ferrous iron can be obtained. When the aim is to obtain an anti-friction layer, generally it resorts to combined phospating processes, in particular of the type zinc-manganese, attempting to increase the ratio of manganese in the obtained deposits.

The mechanical industry requires to precision parts manufacturers a more and more perfect finishing, so that the anti-friction properties of the parts must be increasingly higher. However, in many cases the coatings obtained through the known methods described above do no reach such a finishing quality.

SUMMARY OF THE INVENTION

The invention aims to overcome this disadvantage. This purpose is achieved by a method for applying a phosphate coating on an iron or steel part, characterised in that it comprises:

a first coating step comprising a first phosphating, through which on the base metal of said part a first crystalline layer of iron and manganese phosphates is obtained;

an intermediate step of partial pickling of said first crystalline layer of phosphates, leaving said base metal partially uncovered, and

a second coating step comprising a second phosphating through which, on said partially pickled first crystalline layer of phosphates and on said partially uncovered base metal, a second crystalline layer of iron and manganese phosphates is obtained.

By means of this method according to the invention a coating with a particularly small thickness is achieved, having a particularly fine crystallisation with a high density of crystals by surface unit in the part surface. This leads, on the one hand, to a coefficient of friction much lower than the one that can be obtained by the known coating methods and, on the other hand, to a higher adherence of the coating on the part surface.

In the method according to the invention, advantageously, a known coating method can be applied to carry out these first and second coating steps each one comprising a phosphating; in particular, said coating steps can comprise, typically, subsequent steps of treatment by immersing the part in a series of bathes, being these steps typically as follows: cleaning, degreasing, surface activating, phosphating, rinsing, drying and, optionally, greasing.

The special feature of the method according to the invention is, therefore, the fact of having introduced a partial pickling step applied to the crystalline layer of phosphates which has been obtained by means of a coating method that may be one of the already known, as it has been said, and after that carrying out a new phosphating on said pickled layer. This intermediate pickling stage confers to the surface of the part a particular state that allows the second crystalline layer to be formed with better crystallographic features than the first one. Indeed, after the pickling the surface of the part has such a microstructure and activity that, when applying the second phosphating crystal, grains of phosphates are formed on the nucleation sites of said microstructure, growing with particularly reduced size and in an especially homogeneous form. This advantageous features of the part surface after the intermediate pickling step are precisely due to the fact that said pickling is partial, that is to say that on said surface the rest of the first crystalline layer and the base metal coexist.

By applying the method according to the invention, it is obtained an iron or steel part with an iron and manganese phosphate coating characterised in that said coating has an homogeneous morphology with crystalline formations shaped as needles or columns of a size equal or lower than 2 μm, having said coating a thickness equal or less than 2 μm. Preferably, said coating has an apparent density higher or equal to 3 g/cm³ and lower or equal to 10 g/cm³ and, preferably, said coating has an specific weight higher or equal to 1.5 g/m² and lower or equal to 7 g/m². The specific weight of coating, expressed in g/m², refers to the total mass of the coating deposited by surface unit. The apparent density of coating, expressed in g/cm³, is defined as a ratio between the specific weight of the coating and the coating thickness.

Preferably, said intermediate step of partial pickling consists in a mechanical pickling by erosion in an abrasive medium, by which it is obtained a surface finish suitable for growing the second layer of phosphates with the wished features of a lower grain size and a higher homogeneity. In particular, compared to other forms of pickling such as chemical pickling, mechanical pickling in an abrasive medium has the advantage of providing a partial pickling with a surface finish wherein the completely pickled surface of the base metal, that is, the surface completely free of phosphate crystals of the first layer, coexists with microcavities coated by phosphate crystals of the first layer which are firmly adhered to the metal substrate. This state is particularly advantageous for the growth of the second layer of phosphates with the wished features. Advantageously, said mechanical pickling by erosion in an abrasive medium lasts until at least 15% of the total surface of said first crystalline layer of phosphates had been completely pickled. In this way, a particularly fine and well adhered second layer of phosphates is obtained.

Preferably, said mechanical pickling by erosion in an abrasive medium consists in a rotofinishing using ceramic abrasive with a continuous inflow of an aqueous solution comprising a surfactant and a chemical flocculation agent, having said aqueous solution a pH higher or equal to 6 and lower or equal to 10. By means of this particular method, the intermediate step of pickling provides a surface finish suitable for the grain growth during the second phosphating.

Advantageously, said first and second phosphatings consist in an immersion of said part in an acid solution comprising at least phosphoric acid, metallic acid phosphates, manganese, nickel, iron 2+ and nitrates, having said acid solution a total acid value higher or equal to 58 and lower or equal to 100 and a ratio between total acid and free acid higher or equal to 5 and lower or equal to 7, and being the content of iron 2+ in said acid solution lower or equal to 2.5 g/l. Regarding the concentrations indicated for the phosphating bath, the free acid is defined as the concentration of hydrogen ions corresponding to the first constant of dissociation of phosphoric acid not replaced by metallic ions, being expressed as millilitres or points of solution of NaOH 0.1N which are necessary in order to achieve the neutralisation of said free acid from a sample of 10 ml of bath. By total acid it is understood the millilitres or points of NaOH 0.1N which are necessary in order to achieve the neutralisation corresponding to the equivalence point of secondary phosphates, phosphoric acid and primary phosphates present in a sample of 10 ml of bath, being also this value influenced by the precipitation of different metallic cations present in the bath.

This method is able to advantageously generate a mixed crystalline phase of manganese and iron phosphates of hurealite type, which constitutes said first and second crystalline layers (although the second crystalline layer, as it has been said, has different crystallographic properties regarding the distribution and the grain size).

Preferably, the immersion of said part in said acid solution is carried out by keeping said acid solution at a temperature higher or equal to 95° C. and lower or equal to 98° C. and it lasts while gas is released from the metallic surface of said part, whereby the formation of said step mixed crystalline phase is optimised and a well developed crystalline layer of phosphates is obtained.

The invention further refers to a steel or iron part with a coating obtained according to the method of the invention, having said part the particular features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention are observed from the following description, wherein, without a limiting character, a preferred embodiment is presented, referring to the enclosed figures. The figures showing:

FIGS. 1 a, 1 b, 1 c, 1 d, 1 e and 1 f, images of electronic microscope showing, with a ×1000 enlargement, the part surface after the intermediate step of partial pickling, which in this embodiment example consists in a rotofinishing; the different views correspond to different rotofinishing duration:

FIG. 1 a: 10 minutes

FIG. 1 b: 20 minutes

FIG. 1 c: 30 minutes

FIG. 1 d: 40 minutes

FIG. 1 e: 50 minutes

FIG. 1 f: 60 minutes;

FIGS. 2 a, 2 b, 2 c, 2 d, 2 e and 2 f, images of electronic microscope showing with a ×5000 enlargement the morphology of the second crystalline layer after the second phosphating; the different images are surface views corresponding to the results obtained by rotofinishing durations in the intermediate step of partial pickling;

FIG. 2 a: 10 minutes

FIG. 2 b: 20 minutes

FIG. 2 c: 30 minutes

FIG. 2 d: 40 minutes

FIG. 2 e: 50 minutes

FIG. 2 f: 60 minutes;

FIGS. 3 a and 3 b, images of electronic microscope showing with a ×5000 enlargement, in surface views, the morphology of the crystalline layer of phosphates; FIG. 3 a corresponds to the results obtained after the first phosphating, while FIG. 3 b corresponds to the results obtained after the second phosphating;

FIGS. 4 a and 4 b, images of electronic microscope similar to the ones of FIGS. 3 a and 3 b, respectively, but in cross-sectional view; in this case, the image of FIG. 4 a has a ×8000 enlargement and FIG. 4 b has a ×13000 enlargement;

FIG. 5, a comparative graphic of the thickness of the crystalline layer of phosphates, after the first phosphating and after the second phosphating; the total thickness of the coating e, expressed in μm, is represented in the axis of ordinates; the different points correspond to different measure points in the part surface; the points represented by a triangle correspond to the situation after the first phosphating, whereas the points represented by a filled circle correspond to the situation after the second phosphating;

FIG. 6 a, a comparative graphic of the roughness Rk of the part surface, after the first phosphating and after the second phosphating; the roughness Rk, expressed in μm, is represented in the axis of ordinates; the different points correspond to different measure points in the part surface; the points represented by a triangle correspond to the situation after the first phosphating, whereas the points represented by a filled circle correspond to the situation after the second phosphating.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The method according to the invention is applied, as a non-limiting example, on a soft mechanised steel part subjected to a thermal treatment consisting in quenching and tempering. In this embodiment, said part is a recess for a roller used in pumps for injecting gas-oil at very high pressures and the applied method comprise the following steps:

1) First coating step, consisting in the following operations:

-   -   Alkaline degreasing by immersion in a bath of deionized water         with an additive of trade name “Ardrox 6514” commercialised by         the firm Chemetall, at a temperature regulated between 65° C.         and 75° C. and during a minimum period of time ranging from 5 to         6 minutes;     -   Washing by a bath in water, without shaking, with a continuos         renovation, at a temperature regulated between 35° C. and 45° C.         and during a period of time ranging from 3 to 6 minutes;     -   Activating the surface in order to facilitate the nucleation of         phosphate crystals in the subsequent phosphating step, by means         of a bath in a solution with additives of trade names “Gardolene         V6560A” and “Gardolene V6561B” commercialised by the firm         Chemetall, containing sodium polyphosphates and insoluble         crystals of manganese phosphate, applying an air flow ranging         from 5 to 20 m³/h, at a temperature regulated between 20° C. and         30° C. and during a period of time ranging from 2 to 5 minutes;     -   First phosphating by immersion in an acid solution formed by         deionized water and an additive of trade name “Gardobond G4060”         commercialised by the firm Chemetall, this solution comprising         phosphoric acid, metallic acid phosphates, manganese, nickel,         iron 2+ in a content lower than 2.5 g/l and nitrates, with a         total acid value higher or equal to 58 and lower or equal to 100         and a ratio between total acid and free acid ranging from 95° C.         to 98° C. and during a period of time ranging from 3 to 5         minutes;     -   Washing by a bath in water, without shaking, at a temperature         regulated between 35° C. and 45° C. and during a period of time         ranging from 3 to 6 minutes;     -   Drying by immersion in a bath of hydrofuging oil;         obtaining, after this first coating step, a first crystalline         layer of iron and manganese phosphates.         2) Intermediate step of partial pickling of said first         crystalline layer of phosphates, consisting in a mechanical         pickling by erosion in an abrasive medium by means of         rotofinishing using ceramic abrasive, with a continues inflow of         aqueous solution comprising a surfactant and a chemical         flocculation agent, having said aqueous solution a pH higher or         equal to 6 and lower or equal to 10, and carrying out said         mechanical pickling operation at room temperature and during a         period of time of 30 minutes, being completely pickled, that is,         leaving the base metal uncovered, 60% of the total initial         surface of the first crystalline layer of phosphates.         3) Second coating step, comprising a second phosphating and         being identical to the first coating step, obtaining on said         first crystalline layer of phosphates partially pickled a second         crystalline layer of iron and manganese phosphates.

The morphological features of the second crystalline layer of phosphates mainly depends on the time of mechanical pickling of the first crystalline layer of phosphates, that is, the percentage of surface which is completely pickled, as it can be seen in FIGS. 1 a-1 f and 2 a-2 f. In the case of the part treated in this example, this percentage amounts to 60%. However, depending on the type of part to be treated and on the wished results, a lower or higher time for pickling can be applied in order to obtain a higher or lower percentage of completely pickled surface. It has been observed, nevertheless, that a particularly good result is obtained when this percentage is higher or equal to 15%, even though with a lower percentage satisfactory results, in any case higher than the ones of the methods of the prior art, can be obtained.

The first coating step, as described above, corresponds per se to a method for obtaining a coating by phosphating according to the state of the prior art. FIGS. 3 a, 3 b, 4 a, 4 b, 5 and 6 show that the method according to the invention provides a coating of a much higher quality than the one that can be obtained by a method of the prior art, that is applying simply a first coating step. In particular, in FIGS. 3 a, 3 b and 4 a, 4 b can be observed that the crystallographic features of the coating layer after the second phosphating are completely different from the ones observed after the first phosphating. In fact, after the second phosphating the size of the grain is much lower, crystals are more homogeneous and have a crystalline structure with a morphology shaped as fine needles with a size lower or equal to 2 μm, whereas after the first phosphating crystals had a crystalline structure with a typically polyhedral morphology and a size reaching until 10 μm. In FIGS. 5 and 6 can also be observed that the coating layer obtained after the second coating step has a remarkably lower thickness and a much lower roughness than the layer obtained after the first coating step. The decrease of the crystal size as well as its higher density imply a higher adherence of the coating layer on the part, and at the same time lead to a lower coefficient of friction. In fact, in this example, the coefficient of friction measured in the surface of the treated part after the second coating step is 50% lower to the one measured in the part after the first coating step and before the pickling. 

1. Method for applying a phosphate coating on an iron or steel part, being characterised said method in that it comprises: a first coating step comprising a first phosphating, through which on the base metal of said part a first crystalline layer of iron and manganese phosphates is obtained; an intermediate step of partial pickling of said first crystalline layer of phosphates, leaving said base metal partially uncovered, and a second coating step comprising a second phosphating through which, on said partially pickled first crystalline layer of phosphates and on said partially uncovered base metal, a second crystalline layer of iron and manganese phosphates is obtained.
 2. Method according to claim 1, characterised in that said intermediate step of partial pickling consists in a mechanical pickling by erosion in an abrasive medium.
 3. Method according to claim 2, characterised in that said mechanical pickling by erosion in an abrasive medium lasts until at least 15% of the total surface of said first crystalline layer of phosphates had been completely pickled.
 4. Method according to claim 3, characterised in that said mechanical pickling by erosion in an abrasive medium consists in a rotofinishing using ceramic abrasive with a continues inflow of an aqueous solution comprising a surfactant and a chemical flocculation agent, having said aqueous solution a pH higher or equal to 6 and lower or equal to
 10. 5. Method according to claim 1, characterised in that said first and second phosphatings consist in an immersion of said part in an acid solution comprising at least phosphoric acid, metallic acid phosphates, manganese, nickel, iron 2+ and nitrates, having said acid solution a total acid value higher or equal to 58 and lower or equal to 100 and a ratio between total acid and free acid higher or equal to 5 and lower or equal to 7, and being the content of iron 2+ in said acid solution lower or equal to 2.5 g/l.
 6. Method according to claim 5, characterised in that the immersion of said part in said acid solution is carried out by keeping said acid solution at a temperature higher or equal to 95° C. and lower or equal to 98° C. and it lasts while gas is released from the metallic surface of said part.
 7. Iron or steel part with an iron and manganese phosphate coating, characterised in that said coating has an homogeneous morphology with crystalline forms shaped as needles or columns of a size lower or equal to 2 μm, having said coating a thickness lower or equal to 2 μm.
 8. Part according to claim 7, characterised in that said coating has an apparent density higher or equal to 3 g/cm³ and lower or equal to 10 g/cm³.
 9. Part according to claim 8, characterised in that said coating has an specific weight higher or equal to 1.5 g/m² and lower or equal to 7 g/m². 